John, et Listers,
Too much physics content here for me not to put my two cents in. My
references say that "glass" by definition is a "super-cooled liquid". On
the molecular level it has the same structure, or lack thereof, as liquids;
i.e., no crystal structure; i.e., amorphous. As to the stories about the
old stained glass in centuries old churches flowing, I've just accepted
them as true because it seems to jibe with glass being a liquid. I seem to
recall some stained glass at least looking like it had sagged somewhat when
I was In Europe. Maybe it was just that that's what I expected. Didn't have
my "scientist" hat on at the time.
I remember one time reading an article by a physicist explaining how the
refraction of light by the earth's atmosphere causes the moon appear larger
near the horizon. A few weeks later, I came across an article by a
psychologist trying to explain from a psychological basis why our
perception of the moon makes it seem larger near the horizon. It probably
never occurred to the psychologist to measure the diameter at the horizon
and at the zenith.
Anyway, there is a fundamental difference between glasses and, normal
solids. Glasses flow, solids don't (at least below their yield points). The
only question here is how fast.
Bob
At 10:32 AM 8/25/98 -0400, Carmods@aol.com wrote:
>In a message dated 98-08-24 19:23:07 EDT, larry@netkonnect.net writes:
>
><< They also suggested in storing it upright like they do, and not laying
> on the face, as it may distort from its original shape. I learned years
> ago in physics that glass is a liquid, yes, a liquid. Look at Colonial
> house window panes and you can see the flow, even though it is very,
> very slow.
> >>
>Larry, This is a myth. Due to the manufacturing process of glass, years
ago it
>came out thicker on one edge. It was customary in those times to install the
>glass with the thick side down, leading to the myth. They have found glass in
>old churches and houses where the glass had been originally installed upside
>down and was thicker on the top. It would take thousands of years for window
>panes to noticably creep vertically from compression due to gravity.
>I agree that windshields however, should be stored on end to eliminate the
>high bending stress that will cause creep. This is true many materials.
> John Logan
>
>
Robert L. Palmer
Dept. of AMES, Univ. of Calif., San Diego
rpalmer@ames.ucsd.edu
rpalmer@cts.com
|